Device and method for humidifying a gas flow

ABSTRACT

A device used for humidifying the flow of a gas which is to be humidified, e.g., fed to a fuel cell system. A moist gas, e.g. a moist waste gas from said fuel cell system is used for said humidification, whereby said gas flows together with the gas flow in a humidifying device. The two gas flows are separated from each other by a membrane in the humidifying device. The membrane is exclusively permeable for water vapour. According to the invention, at least one bypass line is provides. At least one of the gas flows can be partially guided via said bypass line around the region of the membrane in the humidifying device. The saturation point can thus be freely adjusted in an advantageous manner in the gas which is to be humidified.

The invention relates to a device for humidifying at least one gasstream, in particular a gas stream flowing to a fuel cell system, of thetype defined in more detail in the preamble of claim 1.

In principle, the prior art discloses two different types of PEM fuelcell systems (PEM=polymer electrolyte membrane). In this context, adistinction is drawn between fuel cell systems having a gas generationdevice and fuel cell systems which are operated directly with hydrogen.In the case of fuel cell systems with a gas generation device, ahydrogen-containing gas is generated in the gas generation device inorder to operate the fuel cells. For this purpose, ahydrocarbon-containing compound, for example alcohol, gasoline ordiesel, together with water and if appropriate air is generallyconverted into a hydrogen-rich gas and carbon dioxide.

In both the fuel cell systems described above, the air fed to thecathode region of the PEM fuel cell, or another oxygen-containingmedium, has to be suitably humidified in order to protect the PEM fromdrying out. In addition, in fuel cell systems having a gas generationdevice, the water balance of the fuel cell system must as far aspossible be continuous, in order on the one hand to ensure that the PEMis moistened and on the other hand to provide sufficient water foroperation of the gas generation device without the water having to beconstantly topped up. The water, stored in particular in liquid form, isthen used for hot-steam reforming or auto thermal reforming of thehydrocarbon-containing compound used to obtain hydrogen in the gasgeneration device.

In the PEM fuel cell itself, an anode space, to which the hydrogen orhydrogen-containing gas is fed, is separated from the cathode space, towhich an oxygen-containing medium, in particular air, is fed, by thePEM, which is usually installed as part of a membrane electrode assembly(MEA). In the fuel cell, in structures which are currently customary,water is simultaneously transferred from the anode side to the cathodeside of the PEM. Likewise, the product water is formed during thereaction of hydrogen and oxygen. This product water is usuallydischarged from the cathode space by an exhaust-gas stream.

U.S. Pat. No. 6,007,931 and U.S. Pat. No. 6,048,383 now describecorresponding processes in which the humid exhaust-gas stream from thecathode space is passed through a humidifier device which has a membranepermeable to water vapor. The water vapor contained in the exhaust-gasstream can in this way pass through the membrane and humidify a gasstream flowing in the humidifier device on the other side of themembrane. This gas stream may in particular be the air supplied in thecathode space.

In this method, a dew point which is substantially dependent on thetemperature of the cathode exhaust gas, the transfer capacity of themembrane and the load point of the fuel cell system is established inthe gas stream that is to be humidified and is flowing to the cathodespace. However, at a relatively high membrane transfer capacity, suchhigh dew points may be established in the gas stream that is to behumidified and is fed to the cathode space that reliable operation ofthe fuel cell is no longer ensured. Instead of useful humidification,the result in practice is “flogging” of the PEM or theelectrodes/catalysts arranged in the region thereof and/or of thecathode space, and the electric power of the fuel cell therefore drops.

Therefore, the object of the present invention is to provide a simpleapparatus of compact design and a method for humidifying at least onegas stream, in particular a gas stream flowing to a fuel cell system, inwhich it is possible to vary the dew point in the at least one gasstream that is to be humidified, and in particular to set it to apredetermined value.

According to the invention, this object is achieved by the featureslisted in the characterizing clause of claim 1.

A method for achieving the above object in combination with theapparatus as claimed in claim 1 results from the characterizing clauseof claim 14.

Particularly expedient uses for the apparatus according to the inventionas described above and optionally the method according to the inventionreferred to above emerge from claims 15 or 16.

The at least one bypass line can be used to influence the volumetricflow on at least one side of the membrane that is permeable to watervapor in a particularly simple and effective way.

For example, it is possible to vary the volumetric flow which is to behumidified and takes up the water vapor. This volumetric flow can then,for example, be mixed with the part passing through the bypass line, sothat the desired dew point can be set in the mixture. It is thereforepossible to vary the dew point by means of a simple device, for exampleby means of proportional control valves or the like.

Alternatively, it is also possible for part of the humid gas to berouted around the membrane through a bypass line. It is in this waypossible to alter the quantity of water vapor provided by influencingthe volumetric flows likewise in a very simple way, for example by meansof a valve device, with the result that it is also possible to vary thehumidification of the gas stream that is to be humidified. In this way,it is also possible by very simple means to influence the dew point inthe gas stream that is to be humidified and is subsequently passed, forexample, to a fuel cell.

According to a particularly expedient refinement of the abovementionedinvention, the humid exhaust-gas stream, after it has flowed through thehumidifier device and/or the bypass line, is routed into a furtherhumidifier device, of comparable design, in order to humidify a furthergas stream. This structure ensures that all the humidity or water vaporwhich is present in the exhaust-gas stream is recovered and madeavailable to the system. This has particularly beneficial effects, forexample in a fuel cell system, on the water balance of this system, sothat there is no need to top up the water in order for the fuel cellsystem to operate.

According to a further highly expedient configuration of the invention,the at least one bypass line is integrated in the humidifier deviceitself.

This results in a very compact, space-saving structure which isparticularly advantageous in particular when the invention is used in afuel cell system in a motor vehicle, boat or the like.

In a refinement of the invention, the apparatus according to theinvention can also be used to dry a gas stream.

This merely requires the “useful gas stream” and “exhaust gas (strippinggas) stream” to be reversed. For example, a humid gas stream can bedehumidified to an extent which can be set accurately by means of astripping gas stream which is initially dry and then downstream of thehumidifier device is humid.

Further advantageous configurations of the invention will emerge fromthe remaining subclaims and are explained in more detail on the basis ofthe exemplary embodiment described below with reference to the drawings,in which:

FIG. 1 shows a fuel cell system in an embodiment according to theinvention;

FIG. 2 shows a possible structure of a humidifier device according tothe invention in the form of an outline illustration;

FIG. 3 shows a structure of the humidifier device shown in FIG. 2 with adevice for varying the volumetric flow through a bypass line;

FIG. 4 shows a further possible structure of a device for varying thevolumetric flow through the bypass line;

FIG. 5 shows a further alternative structure of a device for varying thevolumetric flow through the bypass line;

FIG. 6 shows a further possible structure of a humidifier deviceaccording to the invention in the form of an outline illustration;

FIG. 7 shows an alternative embodiment of the fuel cell system accordingto the invention; and

FIG. 8 shows a further alternative embodiment of the fuel cell systemaccording to the invention.

The following text provides a detailed description of the invention onthe basis of an apparatus for humidifying a gas stream for a fuel cellsystem, but the invention is not intended to be restricted to thisspecific application.

FIG. 1 illustrates a fuel cell system 1. The fuel cell system 1 has atleast one fuel cell 2, which may be constructed as an individual cell orin particular as a fuel cell stack comprising a plurality of individualcells. The fuel cell 2 has a proton-conducting membrane 3, in particulara PEM, which separates an anode space 4 from a cathode space 5 of thefuel cell 2. In the fuel cell 2, electric power is generated in a mannerknown per se from a hydrogen-containing medium, which is fed to theanode space 4 and an oxygen-containing medium, which is fed to thecathode space 5. Neither this electric power nor the way in which thehydrogen-containing medium is generated and/or supplied is of relevanceto the humidification apparatuses presented here.

By way of example, in the fuel cell system 1 illustrated here, anoptional gas generation device 6 is indicated, in which a hydrogen-richgas is generated from a compound which includes carbon and hydrogen,together with water and if appropriate an oxygen-containing medium.However, the mode of operation of the invention can in principle also beimplemented in other fuel cell systems 1, for example in those which aresupplied with hydrogen gas stored in a tank.

In the fuel cell system 1 illustrated here, the product water generatedby the fuel cell 2 will form usually in the region of the cathode space5. This product water is then discharged from the cathode space 5together with an exhaust-gas stream. At the same time, however, thecathode space 5 also requires a defined moisture content or dew pointduring the supply of the oxygen-containing medium, in particular air, inorder to prevent the membrane 3, which is designed, for example, as apolymer electrolyte membrane, from drying out.

Therefore, the exhaust-gas stream from the cathode space 5 is passedinto a humidifier device 7. In this humidifier device 7 there is atleast one membrane 8 which is inherently impermeable to the exhaust gasand permeable to the water vapor contained in the exhaust gas. Membranes8 of this type are known from the prior art and may consist of variouspolymer materials, hollow-fiber membranes or the like. A gas stream thatis to be humidified, takes up the water vapor passing through themembrane 8 and, in the exemplary embodiment illustrated here, is thenfed to the cathode space 5 as humidified feed air flows on the otherside of the membrane 8. A suitable delivery device 9, e.g. a compressor,is located upstream of the humidifier device 7 in order to deliver thisgas stream. The exhaust-gas stream, before it passes from the region ofthe cathode space 5 into the humidifier device 7, is cooled by means ofa heat exchanger 10, so that part of the product water can condense out.This condensed-out part of the product water of the fuel cell and afurther part of the product water of the fuel cell which is entrained inliquid form by the exhaust-gas stream is separated off in liquid formbetween the heat exchanger 10 and the humidifier device 7 in a liquidseparator 11. This water which is separated off in liquid form can thenbe used for other purposes, which will be dealt with in more detail inthe exemplary embodiments explained below.

In a fuel cell system 1 of this type, therefore, it is ensured in asimple and effective way that at least the part of the water which isthe greatest, at least approximately, in the exhaust-gas stream isrecovered. In particular, a part of the water vapor which is present inthe exhaust-gas stream is used for the necessary humidification of a gasstream, in particular the feed air to the cathode space 5. A drawback inthis context is that the transfer of the water vapor and therefore thehumidification of the gas stream that is to be humidified is fixedlypredetermined by the size of the membrane 8. It is desirable to set thedew point to be realised in the gas stream that is to be humidified,which is necessary under certain load conditions in order to prevent anexcessive water content in the region of the cathode space 5, since thiswould be detrimental to the performance of the fuel cell 2.

The fuel cell system 1 illustrated here now provides at least one of twobypass lines 12, 13, in this case indicated by dashed lines, in order toinfluence the dew point of the gas stream that is to be humidified. Inthis context, in principle each of the two bypass lines 12, 13 on itsown is able to realise setting of the dew point in the gas stream thatis to be humidified, by extremely simple means. Furthermore, in additionto each individual one of the bypass lines 12, 13, it is also possibleto provide a combination, i.e. the presence of both bypass lines 12 and13, in the fuel cell system 1. The functioning of the bypass lines isexplained in detail below.

The bypass line 12, which has a device (not shown here) for varying thevolumetric flow which flows inside it (which device can be used to setthe proportion of the volumetric flow of the exhaust gas which flowsthrough the bypass line 12) functions as follows. A part of theexhaust-gas stream which transports the water vapor passes into thebypass line 12, whereas only the remaining part passes into thehumidifier device 7. This makes it possible by extremely simple means tovary the supply of water vapor in the humidifier device 7, so that thegas stream that is to be humidified can only take up the water vaporavailable, and in this way it is possible to set the dew point in thegas stream that is to be humidified by means of the supply of watervapor. This variant has the drawback that part of the humid exhaust gasfrom the fuel cell system 1 shown in FIG. 1 leaves the fuel cell system1 unused, and therefore water also passes to the environment unused.However, this can be avoided by a structure as described below in FIG.8.

The alternative variant (which if appropriate may also be used as anadditional measure) comprising the bypass line 13 provides for only partof the gas stream that is to be humidified to flow through thehumidifier device 7. Downstream of the humidifier device 7, this partcan be mixed again with the gas stream which flows through the bypassline 13 and therefore remains dry. By suitable setting the volumetricratio passing through the bypass line 13, on the one hand, and thehumidifier device 7, on the other hand, it is possible to vary or setthe dew point in the gas stream which in the exemplary embodimentillustrated here then enters the cathode space 5.

FIG. 2 illustrates a cross section through a specific embodiments of thehumidifier device 7. A device for varying the volumetric flows passingthrough the region of the membrane 8, on the one hand, and the region ofthe bypass line 12, 13, on the other hand, is not illustrated in thisfigure, but a device of this type is nevertheless present.

The bypass line 12, 13 (it is of no relevance to the functioning whichof the bypass lines is formed in the embodiment illustrated here) isintegrated in the humidifier device 7. This is particularly expedient ifthe membrane 8 is designed as a bundle of hollow fibers, since thebypass line 12, 13 can then readily be integrated in the bundle as apipeline. However, all other variants of membranes 8 are alsoconceivable. It is also possible for the geometric shape of thehumidifier device 7 and/or of the bypass line 12, 13 to be varied invirtually any desired way.

One of the gas streams then flows from the entry region 14 of thehumidifier device 7 to the exit region 15 thereof. The other flowsthrough further line elements 16, which are only indicated here, intothe region of the membranes 8. If a higher flow pressure loss is assumedfor the gas stream flowing from the entry region 14 to the exit region15 in the region of the membrane 8 than in the region of the bypass line12, 13, the gas stream will predominantly flow through the region of thebypass line 12, 13. On account of the large surface areas of themembranes 8, this is generally always the case.

In order now to be able to control the volumetric flows, the crosssection of the bypass line 12, 13 is altered by a device for varying thevolumetric flow. In FIG. 3, this device is illustrated as a valveplunger 17. As a result of a movement in the axial direction, theremaining inflow or outflow cross section (the direction of flow is ofno importance to the mode of operation) into or out of the bypass line12, 13 can be varied continuously between “closed” and “open”. Theremaining part of the volumetric flow which then no longer flows throughthe bypass line 12, 13 then flows through the region of the membranes 8,where it is humidified or releases the water vapor which it contains.

FIG. 4 illustrates the device in a further alternative embodiment. Thisis suitable for a round or tubular design of the humidifier device 7.The two disks 18 illustrated, with their openings 19, are arrangedconcentrically one immediately behind the other in the direction of flowin the entry or exit region 14, 15. If they are then rotated withrespect to one another, the different degrees of overlap between theopenings 19 result in different regions of the membranes 8 and/or of thebypass line 12, 13 being opened up for medium to flow through them.

FIG. 5 illustrates a further possible form of a device, in which thecross section through which medium can flow is varied by means of adiaphragm disk 20, which is secured eccentrically to the disk 18 and canbe moved into the region of the cross section through which medium canflow. In this case, it is once again possible to use a second disk inaccordance with FIG. 4 to vary the cross section through which mediumcan flow in the region of the membranes 8.

In addition to these devices for varying the cross section through whichmedium can flow which are illustrated here, it is also possible to useall further variants, combinations of conceivable and suitable devices,in particular diaphragms and the like, which can be varied axiallyand/or in terms of their diameter. The particular advantage of thecompact design results with all embodiments which are designed in such away that they can be integrated in the entry or exit region.

FIG. 6 illustrates a further exemplary embodiment of the humidifierdevice 7. As a modification to the exemplary embodiment illustrated inFIG. 2, the bypass line 12, 13 is in this case arranged eccentrically.If the bypass line 12, 13 is then arranged downward in the direction ofthe force of gravity compared to the remainder of the humidifier device7, any water which condenses out and collects in the region of thehumidifier device can be passed through the bypass line 12, 13 in anideal way. The water can then be discharged through the bypass line 12,13 itself or through an optional outlet opening 21 and then madeavailable again to the system, analogously to the water produced in theliquid separator 11.

FIG. 7 shows an alternative variant of the fuel cell system 1 in whichcomparable components are provided with similar reference numerals tothose used in FIG. 1. In the exemplary embodiment of the fuel cellsystem 1 illustrated here, the gas generation device 6 is a requireddevice rather than an optional device, as in the exemplary embodimentshown above. In the case of the fuel cell system 1 in the exemplaryembodiment shown in FIG. 7, only the bypass line 13 for setting the dewpoint in the gas stream that is to be humidified is present, this bypassline operating in accordance with the principle which has already beendescribed above. Moreover, the water which has been separated out inliquid form in the liquid separator 11 is fed back to the gas generationdevice 6 via a line 21.

In the gas generation device 6, this water is reacted together with ahydrocarbon-containing compound, for example gasoline, diesel, alcoholor the like, in a manner known per se to form a hydrogen-rich gas foroperating the fuel cell 2. In addition to the water, which is suppliedto the gas generation device 6 via the line 22, and thehydrocarbon-containing compound, the supply of which is not illustratedhere, an oxygen-containing medium is also fed to the gas generationdevice 6, this oxygen-containing medium being delivered to the gasgeneration device 6 via a delivery device 23 through a furtherhumidifier device 24. This further humidifier device 23 is also offundamentally similar structure to the humidifier device 7. It likewisehas similar membranes 8, which are only permeable to water vapor. Themoisture required to humidify the feed air to the gas generation device6 in this case likewise originates from the exhaust-gas stream, whichdownstream of the humidifier device 7 still contains a certain residualmoisture content, which it releases to the feed air to the gasgeneration system 6 in the further humidifier device 24. In thisstructure, with the bypass line 13 still providing the option of settingthe dew point in the feed air to the cathode space 5, ideal utilizationof the moisture contained in the exhaust-gas stream can be ensured. Itis not necessary to vary the dew point of the feed air to the gasgeneration device 6, since in this case the controlled supply of liquidwater can in any case subsequently be used to set the water levelrequired for ideal reaction of the starting materials, and the level ofwater vapor in the humidified feed air only provides a relatively smallproportion of the water required.

FIG. 8 illustrates a further exemplary embodiment; in this case too, thereference numerals for functionally equivalent components have beenselected to correspond to those used in the previous figures.

The fuel cell system 1 illustrated in FIG. 8 includes the variant usingthe bypass line 12 described above for setting the dew point in the feedair stream fed to the cathode space 5. As has already been mentionedabove, this variant has the fundamental drawback that part of the humidexhaust gas flows around the humidifier device 7 and the moisture whichit contains is therefore in principle lost. In the exemplary embodimentshown in FIG. 8, this drawback is avoided by the proportion of theexhaust-gas stream which flows through the bypass line 12 not beingmixed directly with the exhaust-gas stream downstream of the humidifierdevice 7, but rather the further humidifier device 24 is arranged inbetween. The moisture contained in the exhaust-gas stream which flowsthrough the bypass line 12 can therefore be transferred into the feedair for the gas generation device 6 in the further humidifier device 24,analogously to FIG. 7. In this case once again, analogously to FIG. 7, adelivery device 23 is illustrated, this device 23 being required todeliver the feed air to the gas generation device 6. The fuel cellsystem 1 shown in FIG. 8 now includes a further optional delivery device25. This is or may be required if the pressure in the gas generationdevice 6 is significantly higher than in the region of the furtherhumidifier device 24. In this case, the delivery device 23 would bedesigned as a low-pressure compressor and the delivery device 25 wouldaccordingly be designed as a high-pressure compressor, in order toensure the system pressure which may be required for the gas generationdevice 6.

In the exemplary embodiment illustrated here too, the part of themoisture in the exhaust-gas stream which is approximately the greatestis utilized to humidify the corresponding gas streams that are to behumidified or to provide the water required for the gas generationdevice 6.

Of course, in addition to the exemplary embodiments which have beenillustrated separately here, it is also conceivable to use allconceivable and suitable combinations thereof, as well as their use witha gas generation device 6 or with hydrogen stored in correspondingstorage devices. For all fuel cell systems 1, the water vapor which ispresent in the exhaust gas can ensure ideal humidification of gasstreams that are to be humidified, in particular the feed air to thecathode space 5, while the dew point in this gas stream can be set asdesired. Furthermore, the exemplary embodiments illustrated in FIGS. 7and 8 show possible ways of also recovering the residual moistureremaining in the exhaust-gas stream, for example for operating the gasgeneration device 6, if present.

On account of the particularly compact and robust structure, thehumidification apparatus is particularly suitable for humidification inparticular for fuel cell systems 1 in land, water and air vehicles, andin this context both for fuel cell systems 1 which provide energy fordriving purposes and in particular also for fuel cell systems 1 used asauxiliary power units (APUs).

1-19. (canceled)
 20. An apparatus for humidifying a gas stream, comprising: a humidifier device including at least one membrane permeable to water vapor, wherein the gas stream and a humid gas stream flow through a humidifier device, the a gas stream and the humid gas stream being separated from one another by the at least one membrane; and at least one bypass line configured to route at least part of one of the gas stream and the humid gas stream so that it does not come into contact with the membrane.
 21. The apparatus as recited in claim 20, wherein the gas stream is an inlet gas stream for a fuel cell system and the humid gas stream includes an exhaust gas stream from the fuel cell system.
 22. The apparatus as recited in claim 20, wherein the at least one bypass line is configured to route a first portion of the gas stream and to combined again downstream of the bypass line with the gas stream that flows through the humidifier.
 23. The apparatus as recited in claim 20, wherein the at least one bypass line is configured to route a portion of the humid gas stream.
 24. The apparatus as recited in claim 21, wherein the fuel cell system includes a fuel cell having a cathode space and wherein the gas stream is a feed air for the cathode space.
 25. The apparatus as recited in claim 21, wherein the humid gas stream contains at least a part of the exhaust gases from a fuel cell of the fuel cell system.
 26. The apparatus as recited in claim 21, wherein the at least one bypass line is integrated in the humidifier device.
 27. The apparatus as recited in claim 26, wherein the at least one bypass line is disposed within the humidifier device such that any condensate which collects flows out through the bypass line.
 28. The apparatus as recited in claim 26, further comprising a device for varying a volumetric flow through the at least one bypass line integrated in the humidifier device.
 29. The apparatus as recited in claim 28, wherein the device for varying the volumetric flow includes a valve plunger disposed at one of an inlet and an outlet opening of the bypass line, the valve plunger configured to vary a cross section of the bypass line depending on a distance to the inlet or outlet opening.
 30. The apparatus as recited in claim 28, wherein the device for varying the volumetric flow includes a variable diaphragm that varies a cross section of the bypass line depending on a position and opening diameter of the variable diameter.
 31. The apparatus as recited in claim 28, wherein the device for varying the volumetric flow includes of two discs rotatable relative to one another openings, the device varying a cross section of at least one of the bypass line a flow region to the membrane depending on an angle of rotation of the two disks relative to one another.
 32. The apparatus as recited in claim 28, further comprising a further humidifier device disposed downstream of the device, wherein the humid gas stream is routed into the further humidifier device so as to humidify a further gas stream.
 33. The apparatus as recited in claim 32, wherein the further gas stream is a feed air passing into a gas generation device of the fuel cell system.
 34. A method for humidifying a gas stream, comprising: flowing the gas stream through a humidifier having a membrane permeable to water vapor; flowing a humid gas stream through the humidifier, the gas stream and the humid gas stream being separated by the membrane; routing at least a portion of one of the gas stream and the humid gas stream using a bypass line so that it does not come into contact with the membrane; and varying a quantity of the portion so as to set a predetermined dew point in the gas stream.
 35. The method as recited in claim 34, wherein a humidity of the humid gas stream is reduced in the humidifier.
 36. The method as recited in claim 34, wherein the gas stream and the humid gas stream are in a fuel cell system.
 37. The method as recited in claim 36, wherein the fuel cell system generates electrical energy generator in a land, water or air vehicle.
 38. The method as recited in claim 37, wherein the electrical energy generator is provides a driving energy.
 39. The method as recited in claim 37, wherein the electrical energy generator is an auxiliary power unit. 